The following discussion of implantable devices uses the specific example of the auditory system and cochlear implants. But the present invention is not limited to that specific application and is extensible to other implantable systems and devices, including without limitation brain-related functioning and corresponding cranial implants.
FIG. 1 shows the anatomy of a normal human ear. A normal ear transmits sounds through the outer ear 101 to the eardrum 102, which moves the three bones of the middle ear 103, which in turn excites the cochlea 104. The cochlea, or inner ear, 104 includes an upper channel known as the scala vestibuli 105 and a lower channel known as the scala tympani 106, which are connected by the cochlear duct 107. In response to received sounds, the stapes, a bone of the middle ear 103, transmits vibrations via the fenestra ovalis, (oval window) 114, to the perilymph (cerebrospinal fluid) of the cochlea 104. Vibrations in the cerebrospinal fluid are dissipated out of the fenestra rotunda (round window) 115. As a result, the hair cells of the organ of Corti are excited to initiate chemical-electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain.
Some patients may have partially or completely impaired hearing for reasons including: long term exposure to environmental noise, congenital defects, damage due to disease or illness, use of certain medications such as aminoglycosides, or physical trauma. Hearing impairment may be of the conductive, sensory neural, or combination types.
There are several types of middle- and inner-ear implants that can restore a sense of partial or full hearing. Implants often include various electromagnetic transducers that may function as an actuator, a sensor, and/or a switch. An example of an implant with an electromagnetic actuator is a middle ear implant which mechanically drives the ossicular chain, the three bones of the middle ear that mechanically connect the eardrum to the oval window. Another example of an implant with an electromagnetic actuator is a middle ear implant that mechanically drives the tympanic membrane.
Another type of implant relies on direct electrical stimulation of the nerves in the inner ear. For example, intra-cochlear electrodes can restore some sense of hearing by direct electrical stimulation of the neural tissue in proximity of an electrode contact. These electrodes are typically located on the end of an electrode carrier that is threaded into the cochlea. The electrodes are connected to, for example, an implanted signal processor which communicates with an external signal processor that produces an electrical stimulation signal for the implanted electrodes to stimulate the cochlear nerve.
In order to treat certain inner ear disorders, it is often necessary to deliver therapeutic agents directly into the cochlea. An example of a system for delivering therapeutic agents to the inner ear is a catheter that is inserted into the cochlea via the round window. The end of the catheter might be infused with a therapeutic agent that is released into the cerebrospinal fluid. The catheter might also include a fluid reservoir with a solution of the therapeutic agent that is in fluid communication with the cerebrospinal fluid. Alternatively, the catheter might include a fluid filled lumen containing a solution of the therapeutic agent that is in fluid communication with the cerebrospinal fluid. Delivery of therapeutic agents to the cochlea is described further in U.S. patent application Ser. No. 11/374,505, filed Mar. 13, 2006, the contents of which are incorporated herein by reference.